The present invention relates to an oscillation circuit of the Colpitts-type that employs a ceramic resonator which vibrates in the energy trapping thickness shear slide mode.
FIG. 1 shows a well-known ceramic resonator which vibrates in the energy trapping thickness shear slide mode. This resonator comprises a rectangular ceramic substrate S, an electrode e.sub.1, which extends from an end of one major surface of the substrate S, and another electrode e.sub.2, which extends from another end of the other major surface of the substrate S to partially overlap the electrode e.sub.1. The resonator has an impedance characteristic shown in FIG. 2. The width W and an overlap length .DELTA.L of the electrodes e.sub.1 and e.sub.2 are selected so as to prevent abnormal oscillation and to suppress a ripple R caused in the frequency band .DELTA.F to a minimum.
The width of the frequency band .DELTA.F is varied with the ceramic material. A ceramic resonator having a large bandwidth, i.e., having a large electromechanical coupling factor, is applicable in a wide range excluding a reference clock source for a microcomputer since such a resonator can reduce the rise time of oscillation and oscillate at a low voltage.
When a ceramic material having a large bandwidth is employed for a resonator, however, temperature characteristics of both resonance and antiresonance frequencies Fr and Fa have a negative trend the temperature characteristic of the frequency Fr is steeper in the negative trend than that of the frequency Fa. For example, an impedance characteristic of such a resonator is sifted toward the negative under high temperature, as shown by a dotted line in FIG. 2. When a resonator made of such a material is applied to a Colpitts-type oscillation circuit, therefore, its oscillation frequency Fosc also has a negative temperature characteristic, since the oscillation frequency Fosc is necessarily present between the frequencies Fr and Fa, i.e., in the frequency band .DELTA.F. Particularly, when the oscillation frequency Fosc is closer to the frequency Fr, its temperature characteristic is affected by that of the frequency Fr and is deteriorated.
In order to improve the temperature characteristic of the oscillation frequency Fosc, it is possible to shift the oscillation frequency Fosc toward the antiresonance frequency Fa by reducing load capacitance C.sub.L of the oscillation circuit. In this case, however, a high frequency gain in the circuit is increased so that the circuit could oscillate in an overtone mode to cause spurious results such as a third harmonic wave. Therefore, it is not preferred to change the load capacitance C.sub.L of the oscillation circuit.